Gas generating composition useful for such applications as inflating automotive air bags must meet specific design criteria established by industry standards. For example, gas must be produced at a required rate. The gas that is generated must be innocuous and may contain, if any, only strictly limited amounts of toxic or harmful gases. Moreover, the gas generated must maintain an acceptable burn rate, but the gas generated must not reach a temperature that will injure a vehicle occupant when an air bag inflates or damage the air bag. A gas generating composition useful for this purpose should produce only a limited quantity of particulate materials, and any solids produced must not be harmful or toxic. If solids are produced by the gas generating composition, they should optimally be in the form of a filterable, solid slag. Such solids can be filtered to prevent their interference with gas generating apparatus and their escape into the surrounding environment. Smoke and water soluble particulates from combusted air bag propellants may produce and aggravate pulmonary conditions, especially pre-asthmatic and asthmatic conditions.
The requirements for a gas generator suitable for use in vehicle air bags are clearly very demanding. The generator must burn very fast, on the order of about 30 milliseconds or less, to inflate the air bag. The burn rate must be stable, controllable and reproducible to ensure rapid deployment and inflation of the air bag so that the occupants of the vehicle are not injured and the air bag is not damaged. Ignition of the gas generator must be certain, and the propellant burn rate and inflation time must remain within a required envelope despite extensive exposure of the generator composition to vibration and a wide range of temperatures. The gas generator composition must reliably generate an optimum quantity of innocuous gas during the life of the vehicle, which could be ten years or more. The gas generator composition, moreover, should be insensitive to moisture and must efficiently produce cool, nontoxic, noncorrosive gas at an acceptable temperature that is easily filtered to remove any solid or liquid particles. Combustion of the gas generator composition should ideally produce only water insoluble solid decomposition products that are not respiratory system irritants.
The prior art has proposed a large number of different types of gas generating propellant compositions for use in vehicle air bags and similar safety systems. The majority of the available propellant compositions, while effective gas generators, suffer from various drawbacks. Sodium azide-based gas generating compositions have been those most commonly used in automobile passive restraint systems. Although sodium azide compositions meet most specifications and guidelines for air bag gas generators, they have presented toxicity problems, both alone and with commonly used oxidizers. The solid particulates produced by the combustion of sodium azide-based compositions and non-azide based compositions are water soluble, and may be easily inhaled by vehicle occupants when an air bag deploys and produces smoke. Moreover, the disposal of both deployed and unused air bag inflators with either sodium azide or non-azide gas generants containing poisonous ingredients or water soluble combustion products can present a potential for environmental toxicity.
U.S. Pat. Nos. 5,429,691, 5,592,812 and 5,735,118 to Hinshaw et al. disclose gas generating compositions intended as replacements for sodium azide in vehicle air bags. U.S. Pat. No. 5,429,691 discloses a gas generating composition that includes an oxidizable inorganic fuel, preferably of a transition metal, silicon, boron, aluminum, magnesium, an intermetallic compound, hydrides of metals and mixtures, and an oxidizing agent containing oxygen and a metal. Basic metal carbonates and nitrates are stated to be acceptable oxidizing agents. The reaction for this system does not begin below about 225.degree. F., and theoretical gas yields are disclosed to be comparable to sodium azide systems. The uses of cerium or scandium nitrate or cerium or scandium complex nitrates are not discussed as oxidizing agents or monopropellants. Also, compositions containing azodicarbon-amidine dinitrate and/or diammonium 5,5'bitetrazole organic fuels are not disclosed
U.S. Pat. No. 5,592,812 discloses gas generating compositions formed from complexes of transition metals or alkaline earth metals. The complex includes a cationic metal template, sufficient oxidizing anion to balance the charge of the complex and a neutral ligand with hydrogen and nitrogen. Cobalt is the preferred metal. The disclosed complexes rapidly combust stoichiometrically when contacted with a hot wire or ignitor to produce a metal or metal oxide, nitrogen and water vapor. Again, the use of cerium or scandium oxidizer or monopropellant compounds with or without organic fuels such as azodicarbonamidine dinitrate and/or diammonium 5,5'bitetrazole are not disclosed.
U.S. Pat. No. 5,735,118 discloses a cerium containing compound as a co-oxidizer with other metal complex compounds such as hexamminecobalt nitrate. However, the use of compositions of cerium compounds as singular primary oxidizers with fuels consisting of azodicarbon-amidine dinitrate and/or diammonium 5,5'bitetrazole are not disclosed.
U.S. Pat. No. 5,780,768 to Knowlton et al. discloses a gas generating composition described to be low solids-generating formed from a mixture of a fuel and an oxidizer within a 4% stoichiometric balance. The fuel is selected from the group consisting of guanidine nitrate, nitroguanidine, cellulose, cellulose acetate, hexamene and mixtures thereof. The oxidizer is selected from the group consisting of ceric ammonium nitrate, lithium nitrate, lithium perchlorate, sodium perchlorate, phase stabilized ammonium nitrate, a combination of ammonium nitrate with potassium nitrate, potassium perchlorate and mixtures thereof, such that the combination is a solid solution; a mixture of ammonium perchlorate and at least one alkali metal salt and mixtures thereof; where the fuel is not nitroguanidine when the oxidizer includes ammonium nitrate. This composition may further comprise submicron formed silica to reduce moisture contamination and serve as a processing and powder flow aid and/or binder and may also include an energizing agent. The level of solids produced by the disclosed composition are stated to be less than about 30% and in certain embodiments may be less than about 18%. The most preferred propellant composition, a mixture of guanidine nitrate, ammonium perchlorate and sodium nitrate, fulfills the major objective of this patent, which is to eliminate the air bag inflator filter. The combination of ceric ammonium nitrate is disclosed in combination with guanidine nitrate inflators. Unfortunately, the use of guandine nitrate with ceric ammonium nitrate results in a prohibitively low burning rate. Also, the use of cerium or scandium nitrates or complex nitrates with organic fuels consisting of azodicarbonamidine dinitrate and/or diammonium 5,5'-bitetrazole is not disclosed.
U.S. Pat. No. 5,160,386 to Lund et al. discloses a gas generating composition formed of fuel and a novel oxidizer comprising an inorganic compound with a poly (-nitrito) transition metal complex anion. However, the use of complex nitrates and metal nitrates of cerium or scandium is not discussed with or without the use of azodicarbonamidine dinitrate and/or diammonium 5,5'bitetrazole fuels. Potassium hexanitrocobaltate is the preferred oxidizer. The level of solids produced by the combustion of this composition is not disclosed, however.
In U.S. Pat. No. 5,516,377, Highsmith et al. disclose a gas generating composition based on 5-nitraminotetrazole with an oxidizer that may be an inorganic nitrate or nitrite, a metal oxide, a metal peroxide, an organic peroxide, an inorganic perchlorate, an inorganic chlorate, a metal hydroxide or a mixture of these components. Such low oxygen balance tetrazole systems, however, because of the high concentration of inorganic oxidizers required, tend to produce a high concentration of soluble decomposition products, do not always exhibit optimum ballistic properties and tend to produce higher gas temperatures than desired for vehicle air bags. Again, the use of cerium or scandium based oxidizers with the high oxygen balance fuel, azodicarbonamidine dinitrate, is not disclosed.
A need exists, therefore, for a gas generating propellant composition that can function as either a single component monopropellant or a constituent of a multiple component propellant formulation that combusts to produce an optimum quantity of nontoxic, innocuous, gaseous combustion products and water insoluble solid decomposition products. A need exists, in particular, for propellant formulations which, when combusted, form a minimal to zero concentration of soluble particulate decomposition products to prevent the potential for respiratory system, especially pulmonary, irritation and are effective gas generants.